1. Field of the Invention
The present invention relates generally to microlithographic projection exposure apparatuses comprising an illumination system and a projection lens. More particularly, the invention relates to such apparatuses in which the optical axis of the projection lens does not intersect an illuminated field on a mask that is to be projected.
2. Description of Related Art
Microlithography (also called photolithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. More particularly, the process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus, such as a step-and-scan tool. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed.
A projection exposure apparatus typically includes an illumination system, a projection lens and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a region of the mask with an illumination field that may have the shape of an elongated rectangular slit, for example.
Projection lenses of the catadioptric type do not only contain refractive lens elements but also curved imaging mirrors. The use of curved mirrors is particularly useful in view of the correction of chromatic aberrations and field curvature. In catadioptric lens designs that use polarization selective beam splitter cubes, it is possible to image objects that are centered with respect to the optical axis of the projection lens. However, the use of such beam splitter cubes or similar means has drawbacks, particularly in cases in which the polarization state of the projection light shall not be disturbed by the projection lens.
This has led to the development of catadioptric projection lenses with off-axis fields. With these lenses, the illuminated field on the mask does not contain the optical axis of the projection lens. Such off-axis fields can be projected through the projection lens without the need for beam splitters because the concave mirrors can be positioned such that the projection light bundle impinges obliquely thereon. Thus light cannot be reflected back towards the mask.
In order to achieve good imaging properties, the distribution of the principal rays produced by the illumination system should match as closely as possible the distribution of the principal rays of the projection lens.
Some projection lenses have a homocentric entrance pupil. This may require less refractive power on the entrance side of the projection lens and thus simplify the correction of the field curvature. The illumination system is then designed such that its exit pupil coincides with the entrance pupil of the projection lens. This usually requires only minor adaptations of the focal length of the last group of lens elements contained in the illumination system.
The situation becomes more complicated, however, if the projection lens has not only a homocentric entrance pupil but also an off-axis illuminated field. In principle it would be possible to further increase the geometrical optical flux of the illumination system so that the off-axis field can be properly illuminated. However, this considerably increases the design and manufacturing costs of the illumination system.
U.S. Pat. No. 6,249,382 B1 addresses this problem by proposing to introduce a shift of the optical axis within the illumination system. In a first part of the illumination system between a light source and a field stop, the projection light bundle is centered around the optical axis. In a second section between the field stop and the mask plane, the optical axis is laterally shifted. As a result, the projection light bundle does not contain the optical axis in an intermediate field plane in which a field stop is located. Since both the exit pupil of the illumination system and the entrance pupil of the projection lens are telecentric, this arrangement still ensures a good match between the principal ray distributions.
However, a significant mismatch of the principal ray distributions occurs if the projection lens is not telecentric, but more or less homocentric on its entrance side.